During crash or hard landings of vehicles, heavy payloads exert large g-forces on the mounts connecting them to the structural frame. As a result of those forces, payloads can break loose from these mounts and can cause serious damage to crew, passengers, payload and surrounding structure. Also heavy equipment (e.g. tool boxes, gear boxes, engines), mounted permanently inside of the vehicle, can break out of the hard mounts (Struts, tie rods etc. are some examples of hard tie downs.) and can cause similar damage.
Future heavy lift rotorcraft pose a requirement for payload capacity in excess of 60 kip (267 kN) and feature numerous heavy pieces of equipment. Simply scaling up existing restraint technology may not be the most effective approach because of the potentially high weight penalty associated with reinforcing mounting points in the vehicle structure. Therefore, restraint systems have to be improved and optimized for high effectiveness to weight ratio.
Efficient cargo restraints should be able to absorb the kinetic energy of the cargo, by allowing it to move a limited amount of distance (stroke), without transferring large loads on the mounting structure. Similarly, hard tie downs should be able to absorb kinetic energy of heavy equipment during crash sequences. These two applications differ substantially in the available distance that the cargo/equipment can move. Current payload restraint systems typically lack any load limiting capabilities. Also the hard tie downs fail to limit the load. Forces in the device continue to rise until cargo movement ceases or the device and/or attachment point fails. There is a need for energy absorbing tie downs which can limit the loads.
The most important of the issues which have to be addressed are the design constraints, such as allowable space for the cargo to move or strength of mounts etc. Generally, the space available to move parallel to the floor of the cabin is more than the space available vertically towards the ceiling. However, permanently fixed equipment, including transmission parts which are very confined in their fixture, have a requirement of very low stroke. Thus these devices have to be scaled in accordance with the design constraints.
For many applications, weight constraints associated with safety equipment are severe. Ease of mounting inside the cabin is another qualifying criterion.
Over a past few decades composites have shown considerable importance in terms of usage in structural applications. Due to their superior strength-to-weight ratio and design flexibility compared to metals, composite structures are excellent replacements for metal parts in various applications. Due to available multi dimensional parametric design space, in terms of type of fiber, type of matrix, fiber volume fractions, fiber angle and number of plies, a composite laminate can be tailored with ease for meeting wide range of design requirements. By exploiting these anisotropic properties, elastic coupling effects can easily be incorporated in the composites. One such important effect is extension-twist coupling. Rehfield et al. (1988) have provided an approach to obtain the stiffness coefficients of a composite tube in their study of thin walled closed cross section composite beams. (See: Rehfield, L. W.; Atilgan, A. R.; Hodges, D. H., “Non classical behavior of thin walled composite beams with closed cross sections” Presented at American Helicopter Society National Technical Specialists' Meeting on Advanced Rotocraft Structures, Oct. 25-27, 1988, Williamsburg, Va.).
Nampy and Smith (2005) have studied the extension twist coupling of flexible matrix composite box beam structures. (See: Nampy, S. N., Smith, E. C., Shan, Y., and Bakis, C. E., “Extension-Twist Coupled Tiltrotor Blades Using Flexible Matrix Composites”, Presented at the Structures and Survivability Specialists' Meeting, Williamsburg, Va., Oct. 25-27, 2005). This coupling gives rise to twisting of composite tubes on application of axial force. Due to this, tubes can apply tangential loads on the attachments holding them circumferentially. The rate of twist can be variable depending upon the above mentioned parameters, fiber angle in particular.
Hagon et al. (2008) have shown that energy can be absorbed by the mechanism of stitch rupture. When threads in the stitches get loaded in tension, they store elastic energy. Upon stitch rupture, these threads break and dissipate the energy (see Hagon, M. J. et al. “Energy-Absorbing Textile Devices for Heavy Cargo Restraints”, Presented at the American Helicopter Society 64th Annual Forum, Montreal, Canada, Apr. 29-May 1, 2008).
The Specific Energy Absorption (SEA) of such so-called stitch ripping devices (SRDs) can be substantially higher than currently used load limiters (wire bender etc.). However, such SRDs are not particularly effective in low stroke applications, where allowable space for cargo/equipment is limited.
Many energy absorption devices rely upon crushing or buckling of the tube or other structure. Examples of such devices are disclosed in U.S. Pat. No. 5,035,307 to Sadeghi et al, U.S. Pat. No. 5,914,163 to Browne, and U.S. Pat. No. 6,949,282 to Obeshaw. U.S. Pat. No. 7,238,250 discloses an energy absorbing structure that cracks while absorbing energy. Bansemir et al. disclose an energy absorbing system in U.S. Pat. No. 6,886,779 in which a sacrificial element is disposed to be uncoupled from a transverse load path of the transverse force-absorbing guide element, and is configured to be deformed and destroyed by a relative movement between connection structures upon application of predetermined maximum load. Finally, the energy absorbing composite tube disclosed by Thayer in U.S. Pat. No. 6,601,886 has a wedge that is forced against a tubular structure causing delamination and thereby absorbing energy.
There is still need for advancement in the area of novel load limiter devices, which can have comparable or better SEA than currently used load limiters, and can be variable in terms of available stroke to length ratio.